Thats a lot of questions do some reading to get a better idea, but PVC is fine so is ABS.
It's best to drill your tank and have an overflow.
Flow depends on what you want to keep but for a return pump on a 90 you will want to run 800 to 1800 gallon per hour. OA or JL will have them.
That is just my opinion though. 
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150 Gal system 3'x3' 400W M/H, Bekett skimmer, Dart return,1/4 HP Chiller
180 Gal Drop tank, LED lights, Bubble master 250 skimmer,Hammerhead on a closed loop, Speed wave return.
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